1. Field of the Invention
The present invention relates, in general, to communication systems, and more particularly, to a switchable wavelength router for wavelength division multiplex (WDM) optical communications.
2. Statement of the Problem
Optical wavelength division multiplexing has gradually become the standard backbone network for fiber optic communication systems. WDM systems employ signals consisting of a number of different wavelength optical signals, known as carrier signals or channels, to transmit information on optical fibers. Each carrier signal is modulated by one or more information signals. As a result, a significant number of information signals may be transmitted over a single optical fiber using WDM technology.
Despite the substantially higher fiber bandwidth utilization provided by WDM technology, a number of serious problems must be overcome, for example, multiplexing, demultiplexing, and routing optical signals, if these systems are to become commercially viable. The addition of the wavelength domain increases the complexity for network management because processing now involves both filtering and routing. Multiplexing involves the process of combining multiple channels (each defined by its own frequency spectrum) into a single WDM signal. Demultiplexing is the opposite process in which a single WDM signal is decomposed into individual channels. The individual channels are spatially separated and coupled to specific output ports. Routing differs from demultiplexing in that a router spatially separates the input optical channels into output ports and permutes these channels according to control signals to a desired coupling between an input channel and an output port.
The Applicants' co-pending U.S. patent application Ser. Nos. 08/685,150 and 08/739,424 (Kuang-Yi Wu et al.) teach two independent methods for high performance signal routing (Ser. No. 08/685,150) and wavelength de-multiplexing (Ser. No. 08/739,424). In Ser. No. 08/685,150, new structures for realizing optical switches (routers) are disclosed that achieve very high extinction ratio operation. However, these switches are wavelength independent. In Ser. No. 08/739,424, a system is disclosed to provide the functions of wavelength de-multiplexing and routing. However, this single stage design relies primarily on the filter design. The transmission function of the filter has to be close to an ideal square flat-top to realize the desired low crosstalk operation.
3. Solution to the Problem
The present invention combines the two architectures and concepts presented in the above-cited patent applications to create a switchable wavelength router. This new structure employs double stage filters that can obtain a better (purified) pass-band transmission, and incorporates a fault-tolerant structure (similar to that disclosed in Ser. No. 08/685,150) that results in low crosstalk between channels.